ITCH: Individual Testing of Computer Homework for Scratch Assignments.

In this paper, we describe ITCH, an automatic testing system for projects in the Scratch programming language. While Scratch lacks traditional input/output mechanisms, ITCH overcomes this by exploiting the persistent saved project state to embed ITCH-specific lists that retain project output. In addition, the ITCH system replaces Scratch user input commands with automatically assigned test values. ITCH can gather variable state from running code, redirect output to a file, and capture drawings done by a Scratch project to save in a report format. The ITCH system has been successfully used in a large computer science explorations class where small practice code questions are assigned before extending work to larger, more creative projects without further burdening course graders. Examples of course projects and the ITCH-tested practice questions are given.